The cost of comminuting and processing ore in the mining industry is determined in part by the cost of the consumable wear surfaces and parts necessary to comminute the ore. To lower the operating costs associated with comminuting processes, it is desirable to increase the life of the comminuting media.
In a typical ore processing arrangement, large pieces of rock or ore must be broken into smaller pieces to liberate the valuable mineral constituents. As a representative example of one method of comminuting ore, large pieces of ore are moved into an enclosed tubular housing known as a grinding mill which rotates the ore. The tubular housing typically includes a plurality of wear resistant plates or elements attached to the interior of the housing to form a liner therein. The rotation of the mill causes the ore to impact on itself and on the liner of the mill, causing break-up of the ore.
In addition, loose comminuting elements are often added to the grinding mill to increase the rate of disintegration of the ore. These elements are steel spheres, rods, cones or the like which rotate within the mill with the ore, pounding the ore and increasing its rate of disintegration.
The comminuting elements must be extremely durable so that when they impact one another, the mill liner and the ore, they do not themselves break apart or wear at an excessive rate. It is desirable for the comminuting elements to wear very slowly in order to increase their useful life. The slower the spheres, rods or other members wear, the less often they must be replaced, thus lowering the cost of the comminuting operation.
The wear resistance of a steel is tied, at least in part, to its microstructure. It is known that martensitic steels exhibit low rates of abrasion wear, as compared to steels having another microstructure, such as pearlitic or stable austenitic steels. The microstructures of steels may be quite complex, but generally consist of one or more phases or phase mixtures, to wit, martensite, austenite, ferrite, carbide, pearlite, and bainite.
As a result of the difficulties surrounding the obtaining and identifying of particular steel microstructures, however, the hardness of a steel has generally been used as the determinant for use of the steel as a comminuting media. In particular, it has generally been taken as a "rule of thumb" that the wear resistance of a steel increases with increasing hardness. It has, therefore, been the conventional wisdom that comminuting elements should be formed from high hardness steel.
Typically, high hardness in steels is attained by increasing the carbon content and heat treating the steel, typically by using an austenitizing and quenching treatment, in such a manner as to form a high amount of martensite. Martensite is a very hard but very brittle phase. As a result, comminuting media comprising martensitic steel has the disadvantage that it may spall and chip.
As one means for increasing the spalling resistance of the martensitic steel, the steel may be given a subsequent heat treatment called tempering. Tempering of a martensitic steel reduces its brittleness, increasing its "toughness" or ability to withstand impact loading without spalling and chipping. Tempering, of course, typically reduces the hardness of the steel, and presumably its abrasion wear resistance. Tempering also adds another step to the process of making the steel, increasing the cost of the end product.
It is desirable to create a steel which is useful to form comminuting wear surfaces. The steel preferably has the wear resistance of high hardness steels such as high carbon martensitic steel, and yet is sufficiently ductile to minimize failure by cracking and spalling under impact loading.